PROJECT SUMMARY Despite modest improvement in patient outcomes from recent advances in pharmacotherapy targeting fibrogenic signaling pathways, Idiopathic Pulmonary Fibrosis (IPF) remains a major unsolved clinical problem. One reason for this is that currently available anti-fibrotic agents slow but do not arrest fibrotic progression. To fully arrest fibrotic progression, its obligatory drivers need to be identified. We have made several recent discoveries studying primary cells and ECM from IPF patients that illuminate a path forward toward understanding mechanisms driving the relentless progression of fibrosis following disease initiation. We learned that fibrosis progression involves cell-intrinsic/autonomous mechanisms. Cell-autonomous fibrogenicity was established by our discovery of fibrogenic mesenchymal progenitor cells (MPCs) in the lungs of patients with IPF and that S100A4 mediates their fibrogenicity. Our data indicate that fibrogenic mesenchymal cells residing in a fibrogenic ECM is a key driver of fibrotic progression in the human IPF lung. We have recently discovered that the sentinel morphologic lesion of IPF, the fibroblastic focus, is a polarized structure with an active fibrotic front that contains IPF MPCs, their transit amplifying progeny and activated macrophages residing in a hyaluronan (HA)-rich extracellular matrix (ECM) that supports their pathological properties. In contrast, the core of the fibroblastic focus is a collagen-rich region containing non-cycling myofibroblasts actively synthesizing and depositing ECM. In this proposal we will: i) examine the role of nuclear, cytoplasmic and extracellular S100A4 in regulating the fibrogenicity of IPF MPCs and their progeny; ii) define the key components of the fibrogenic niche microenvironment that regulate S100A4 function; and iii) identify subgroups of phenotypically distinct MPCs within the fibrogenic niche that cooperate to drive fibrotic progression. Two Aims are proposed: In Aim 1 we will determine the molecular mechanism by which the HA-CD44 axis regulates nuclear S100A4 function and MPC self-renewal and identify subgroups of IPF MPCs based on SSE4 and HA cell surface expression; in Aim 2 we will determine the role of cytoplasmic S100A4 in acquisition of a motile phenotype as IPF MPCs differentiate to IPF fibroblasts; and examine the role of IPF MPC progeny-derived extracellular S100A4 in driving fibrotic progression. New knowledge about subgroups of fibrogenic MPCs, the cells that support them within the active fibrotic front, and fibrogenic signals in the microenvironment; has the potential to greatly advance the design of therapeutics that fully arrest fibrotic progression and even reverse established fibrosis. !